There are many oil-bearing subterranean formations from which the resident oil cannot be recovered in economic quantities by primary recovery techniques. In these formations, secondary recovery techniques must be employed to enable the oil to be produced in economic quantities. One of the secondary recovery techniques which has been found to be well-suited for use in these formations is known generally as steam stimulation. In this technique, steam is injected into the formation for a period of time until the formation is heated sufficiently well so that the viscosity of the oil contained therein is reduced to a degree that it may be readily produced.
Fundamentally, water can exist as either a gas or a liquid under saturated conditions. Wet steam can contain both gas and liquid components, known to those skilled in the art as two-phase flow. A common method of expressing the quantities of each phase, known as quality, is the ratio of the mass flow rate of the gas phase to the total mass flow rate, expressed as a number less than one or as a percentage. Another expression of steam quality is the use of the ratio of vapor to liquid.
In thermally enhanced oil recovery projects it is common to employ a high quality, two-phase steam which may be prepared for convenience at a central steam generating facility. As is well known to those skilled in the art, the practice of utilizing a high quality, two-phase steam is necessitated by the use of low quality, brackish waters having at least a moderate level of dissolved solids. To prevent deposition of salts on the surface of the steam generator tubes, it is necessary to retain part of the flow in a liquid state in order to maintain the solids and other impurities in solution. As can be appreciated, should the steam so generated be required to be distributed to a plurality of injection wells from a single generator output line, it is essential that this plurality of individual flows be maintained at a consistent and desirable vapor-to-liquid ratio. The problem which exists in the distribution of a two-phase mixed stream of vapor and liquid to a plurality of locations is that without special provisions, the vapor and liquid components will not divide into flows of uniform vapor-to-liquid ratio.
Several attempts have been made to provide an apparatus for distributing a two-phase mixed stream of vapor and liquid. For example, U.S. Pat. No. 3,899,000 provides a closed vessel structure for the separation of a two-phase vapor-liquid mixture into two or more individual flows. The vessel disclosed is mounted vertically and provided with a top inlet and two or more bottom outlets. A flat, horizontal baffle is used to divert the inlet flow from the open ends of the outlets. The axis of the inlet and the axis of the outlets are substantially parallel so that the flow of the fluid is axially through the elongated vessel. It is taught that the vapor-to-liquid ratio is maintained by using the outlets as standpipes and the vessel as a reservoir. Once sufficient liquid collects in the bottom of the vessel, it can overflow the side outlets in the standpipes and liquid will be added to the vapor flowing out of the outlets.
U.S. Pat. No. 4,269,211 discloses a method for equalizing the steam quality in a plurality of branch lines of a high pressure steam pipeline. Also disclosed is a steam manifold distribution system which includes a mechanism for retracting a perforated baffle plate into a pressure equalizer chamber for removal, repair or replacement of the baffle plate. The pressure equalizer chamber of U.S. Pat. No. 4,269,211 may be positioned on and fixedly attached to a tee joint in the field in any position between and coaxial with one of the branch lines of the tee joint and perpendicular thereto.
U.S. Pat. No. 4,505,297 discloses an apparatus for dividing a single stream vapor-liquid mixture into a plurality of individual streams while maintaining a similar vapor-to-liquid ratio in the individual streams. The apparatus taught comprises a closed vessel having a central inlet in the top for the inlet feedstream and a plurality of outlets in the side of the vessel for the individual streams. A frustrum-shaped diverting member is mounted in the center of the vessel to divert the flow of the single feedstream into the individual streams. A bottom drain is disclosed for use in removing any liquid that is separated from the vapor-liquid mixture.
U.S. Pat. No. 4,800,921 teaches the utilization of a gravity influenced liquid distribution system in an annular flow regime within a substantially horizontal header which receives a liquid vapor mixture from a supply line and divides that single stream into a plurality of streams for distribution through a branchline to a nearby site. The header employed is substantially horizontal, with each branchline connected to the periphery of the header further downstream and relatively lower on the periphery of the header than the preceding branchline.
Application Ser. No. 526,475, filed on May 21, 1990, the inventor of which is a co-inventor of the present invention, discloses a steam manifold and distribution system capable of uniformly distributing steam throughout a field through a plurality of steam distribution lines. The manifold includes a flow disperser, at least two hollow runners in fluid communication with the outlets of the flow disperser, a substantially toroidal manifold shell in fluid communication with the runners, the manifold shell defining a manifold chamber, and a plurality of distribution ports spaced about the substantially toroidal manifold shell, each distribution port in fluid communication with the manifold chamber. As taught therein, the hollow runners direct the two-phase mixed stream of vapor and liquid into the manifold chamber perpendicular to the plurality of distribution ports spaced about the substantially toroidal manifold shell. Application Ser. No. 526,475, is hereby incorporated by reference for all that it discloses.
It has now been found that although the manifold disclosed in Application Ser. No. 526,475 provides a design which aids in the uniform distribution of a two-phase mixed stream of vapor and liquid, thermal stresses encountered within the manifold may be higher than desired. When encountered, these thermal stresses may eventually lead to stress cracking in regions of high stress concentration.
Therefore, there exists a need for an improved steam manifold and distribution system capable of uniformly distributing steam through a plurality of steam distribution lines which does not experience high thermal stressing in use.